The story of those who made possible the first human flight into space, but remained behind the scenes. The heroes of the film are Semyon Kosberg and other engineers who worked in the Voronezh design bureau on the third stage of the rocket engine, without which the Gagarin spacecraft would not have entered orbit on April 12, 1961. Memories of Viktor Skachilov, the only surviving member of Kosberg's team, and archive photos are intertwined with animation and an inspired soundtrack created specifically for the film. "Kosberg worked" is about hard work for the sake of a great dream, about scientific breakthroughs and people who do the incredible.